A subscriber identity module (SIM) card may be inserted into a mobile communication device to differentiate one wireless communication service subscriber from a different subscriber and to differentiate one mobile network operator from another. The SIM card may store data for establishing and maintaining wireless communication links between a mobile communication device and cell sites or cell towers. For example, the SIM card may store authentication keys for granting wireless service to the mobile communication device. The SIM card may store roaming rules and data. A SIM card may be a removable card with processing and memory capability but without its own power source.